This is a continuation-in-part of co-pending International Application PCT/GB2005/001855, filed May 13, 2005, published Dec. 1, 2005, as Publication No. 2005/114258, now pending, and claiming priority to Application Serial No. GB 2 414 299 A, filed May 21, 2004, in the United Kingdom, which is hereby incorporated by reference for all purposes as if set forth verbatim herein.
This disclosure herein is related to the disclosure in U.S. application Ser. No. 11/457,125, entitled “Time Lapse Marine Seismic Surveying Employing Interpolated Multicomponent Streamer Pressure Data”, filed on an even date herewith in the name of Johan Olof Anders Robertsson and commonly assigned herewith.
1. Field of the Invention
The present invention pertains to marine seismic surveying and, in particular, to time lapse surveys in a marine environment.
2. Description of the Related Art
Seismic exploration involves surveying subterranean geological formations for hydrocarbon deposits. A survey typically involves deploying acoustic source(s) and acoustic sensors at predetermined locations. The sources impart acoustic waves into the geological formations. The acoustic waves are sometime also referred to as “pressure waves” because of the way they propagate. Features of the geological formation reflect the pressure waves to the sensors. The sensors receive the reflected waves, which are detected, conditioned, and processed to generate seismic data. Analysis of the seismic data can then indicate probable locations of the hydrocarbon deposits.
Historically, seismic surveys only employed pressure waves and the receivers detected any passing wavefront. This sometimes leads to difficulties in processing. The art has therefore recently begun moving to “multicomponent” surveys in which, for example, not only is the passing of a wavefront detected, but also the direction in which it is propagating. Multicomponent surveys include a plurality of receivers that enable the detection of pressure and particle velocity or time derivatives thereof (hereafter referred to as “particle motion sensors”). In so-called dual sensor towed streamers, the streamer carries a combination of pressure sensors and particle motion sensors. The pressure sensor is typically a hydrophone, and the particle motion sensors are typically geophones or accelerometers. Knowledge of the direction of travel permits determination, for example, of which wavefronts are traveling upward and will yield useful information and which are traveling downwards and will yield undesirable information if confused with upwards traveling waves.
Some surveys are known as “marine” surveys because they are conducted in marine environments. Note that marine surveys may be conducted not only in saltwater environments, but also in fresh and brackish waters. Marine surveys come in at least two types. In a first, an array of streamers and sources is towed behind a survey vessel. In a second type, an array of seismic cables, each of which includes multiple sensors, is laid on the ocean floor, or sea bottom, and a source is towed from a survey vessel.
Accurate knowledge of source and sensor positions is important to the accuracy of the analysis. In land surveys, accurate positioning is not particularly difficult because environmental conditions are usually relatively stable. Sources and sensors can be readily positioned where desired and, once placed, they usually do not shift to any great degree. Marine surveys, however, are different altogether. In marine surveys, many factors complicate determining the position of the sensors, including wind, currents, water depth, and inaccessibility.
One increasingly common marine seismic survey technique is known as “time lapse seismic surveying.” This technique essentially repeats earlier surveys over time to reveal changes in reservoirs of hydrocarbon deposits. One way to do this is to position the acoustic source(s) and receivers as close as is reasonably practicable to the positions of corresponding acoustic source(s) and receivers in the earlier survey(s). However, as was noted above, this is very difficult to do for marine surveys. A specification of how good the positions need to be in a time lapse survey is often defined in the contract with the client. When the positions of the acquired data do not fit the specifications, then the survey needs to reshoot some of the sail-lines, which is referred to as “infill”. Shooting infill can be a very time-consuming and costly effort in a time lapse program.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.